In CT X-ray imaging of a patient, X-rays are used to image internal structure and features of a region of interest (ROI) of the patient's body. A multislice CT scanner generally comprises an X-ray source that provides a cone shaped X-ray beam radiated from a focal spot of the X-ray source and an X-ray detector array comprising a plurality of closely spaced rows of X-ray detectors that face the X-ray source. In third generation multislice scanners both the X-ray source and detector array are mounted in a rotor of a gantry. In fourth generation multislice scanners the X-ray source is mounted to the rotor but the X-ray detector array, in which detectors comprised in the array form complete circles of detectors, is mounted to the gantry.
A patient being imaged with the scanner is generally supported on a bed which is moved axially along a z-axis to position the ROI in a field of view (FOV) of the scanner located inside the rotor, between the X-ray source and detector array. The rotor is rotatable around the z-axis so as to position the X-ray source, and in third generation CT scanners the detector array, at different cone beam view angles around the patient, from which view angles the X-ray source illuminates the ROI with X-rays. Measurements of intensity of X-rays from the X-ray source that pass through the patient's body at the different view angles provide measurements of attenuation of the X-rays for different attenuation paths through the body. The attenuation measurements are used to image the ROI.
As the rotor is rotated, motion of the X-ray source focal spot occurs in a plane, hereinafter referred to as a “rotation plane”, generally perpendicular to the z-axis. A vertex angle of a fan shaped cross section of the cone beam in the rotation plane is a “fan angle” of the cone beam. A vertex angle of a fan-shaped cross section of the cone beam in a plane perpendicular to the rotation plane that passes through the focal spot and the z-axis is a “cone beam angle” of the cone beam. The cone beam is substantially symmetric relative to the rotation plane and the rotation plane bisects the cone beam angle of the cone beam. To match the symmetric cone beam, the rows of the X-ray detectors in the detector array are symmetrically disposed relative to the rotation plane i.e. each row has a mirror image row in the rotation plane.
For a given size fan angle, the cone beam volume and size of the detector array, and for a given size of X-ray detectors in the detector array, the number of detector rows in the array, are limited by the cone beam angle. The cone beam angle in turn is limited inter alia by an effect referred to as a heel effect.
Let an angle relative to the rotation plane of a path along which X-rays radiated from the X-ray source focal spot propagate be referred to as “declination angle”. It is convenient to define declination angles to be positive on one side, a positive side, of the rotation plane and to be negative on the other side, a negative side, of the rotation plane. The heel effect hardens and decreases intensity of X-rays emanating from the X-ray source as the magnitude of the declination angle increases on one side, arbitrarily the negative side, of the rotation plane. The hardening and intensity reduction is a result of a configuration of an anode comprised in the X-ray source onto which an electron beam is focussed to generate X-rays. As a result of the configuration, for increasingly negative declination angles, as X-rays generated in the anode leave the target, they encounter on the average a greater amount of material from which the anode is formed. As the amount of anode material that the X-rays traverse increases, attenuation and hardening of the X-rays increase, generating thereby the heel effect.
The hardening and drop in intensity as a function of decreasing declination angle determines a minimum declination angle, hereinafter a “heel effect angle” for X-rays below which intensities and energies of X-rays are generally not effective for CT imaging. As a result of the symmetry of the cone beam relative to the rotation plane, a maximum positive declination angle for X-rays is equal to the magnitude of the heel effect angle. The cone beam therefore has a cone beam angle equal to about twice the magnitude of the heel effect angle. The heel effect therefore limits the cone beam angle, and thereby a volume of the cone beam, a total number (for a given detector size) of detector rows in the scanner and a volume of a patient for which CT scan data can be simultaneously acquired.
In order to provide faster CT imaging of patients and enable imaging of moving organs of the body with improved temporal resolution, for example cardiac imaging, there is a need to increase the volume of a patient for which data is simultaneously acquired by a multislice scanner.